内在 (nèi zài) is an adjective meaning 'inner, internal' or 'intrinsic', often used for abstract qualities (e.g., inner world, internal logic). Unlike 内 (nèi), which is a noun meaning 'inside', 内在 always describes a characteristic. It is broader than 固有 (gù yǒu 'intrinsic'), which stresses essential, inseparable traits, and differs from 天生 (tiān shēng) / 先天 (xiān tiān), which refer to qualities present at birth.